1. Field of the Invention
The present invention relates to alloys, in particular for use as heat- and corrosion-resistant coatings, if appropriate in combination with other types of layers, for components which are exposed to high temperatures and corrosive gases. The present invention also relates to processes for producing coatings from alloys of this type in particular on substrates formed from superalloys, and to the use of coatings of this type in particular for coating components of gas turbines or jet engines.
2. Brief Description of the Related Art
Numerous works have dealt with analysis of the thermodynamic stability, mechanical properties and corrosion resistance of the Al—Ru and Al—Ni—Ru B2 alloys. The B2 phase is an ordered cubic phase AB in which the A atoms are arranged at the corners of the unit cell and the B atoms are arranged in the center of the unit cell (cP2- CsCl structure type). B2 Al—Ru is a thermodynamically stable phase which melts congruently at the very high temperature of 2050° C. (cf. in this respect Massalski T. B. (Editor), Binary alloy phase diagrams, 2nd edition OH: ASM International, (1990), 1, 203) and contains a significant amount of aluminum, which is required in order to form the layer of aluminum deposits which provides protection against oxidation. Surprisingly good mechanical properties, including satisfactory mechanical formability at room temperature, are other attractive properties of the Al—Ru B2 phases (cf. in this respect U.S. Pat. No. 5,152,853; Fleischer R. L., (1991), Metall. Trans. A, 22A, 403; U.S. Pat. No. 5,011,554; Wolff I. M., Sauthoff G., (1996), Metall. And Mater. Trans. A, 27A, 2642). Moreover, it has been reported that the oxidation resistance of B2 ruthenium aluminide can be significantly improved by an alloy with a small proportion of yttrium (up to 1 atomic %) and chromium (up to 5 atomic %) (cf. in this respect McKee D. W., Fleischer R. L., (1991), Mat. Res. Soc. Symp. Proc., 213, 969; and Wolff I. M., Sauthoff G., Cornish L. A., Steyn H. DeV., Coetzee R., (1997), Structural Intermetallics, ed. Nathal D. B. et al, 815).
Currently the only intermetallic B2 compounds whose thermal conductivity has been investigated are the following: AlNi, AlFe and AlCo (cf. in this respect Terada Y., Ohkubo K., Nakagawa K., Mohri T., Suzuki T., (1995), Intermetallics, 3, 347; and Reddy B. V., Deevi S. C., (2000), Intermetallics, 8, 1369).
The Al—Ni—Ru phase diagram has been investigated at different temperatures (cf. in this respect Tsurikov V. F., Sokolovskaya E. M., Kazakova E. F., (1980), Vestnik Moskovskogo Univ. Khim., 35(5), 113; Petrovoj L. A., (1985), Diagrammy Sostoyaniya Metallicheskikh System, ed. N. V. Ageeva, VINITI, Moskau, 30, 323; Chartavorty S., West D. R. F., (1985), Scripta Metall., 19, 1355; Chartavorty S., West D. R. F., (1986), J. Mater. Sci., 21, 2721; Homer I. J., Hall N., Cornish L. A., Witcomb M. J., Cortie M. B., Boniface T. D., (1998), J. Alloys and Compds, 264, 173; Homer I. J., Cornish L. A., Witcomb M. J., (1997), J. Alloys and Compds, 256, 221; Hahls J., Cornish L. A., Ellis P., Witcomb M. J., (2000), J. Alloys and Compds, 308, 205). The information available in connection with these studies varies noticeably. Tsurikov et al. report two B2 phases, which are based on AlNi and AlRu, in a ternary system at 550° C. (cf. in this respect Tsurikov V. F., Sokolovskaya E. M., Kazakova E. F., (1980), Vestnik Moskovskogo Univ. Khim., 35(5), 113), asserting that AlRu (β1) is able to dissolve up to 8 atomic % of nickel, and AlNi (β2) is supposed to be able to dissolve up to 5 atomic % of ruthenium. By contrast, Petrovoj found an unlimited solubility between the two B2 phases at 800° C. within the ternary system (cf. in this respect Petrovoj L. A., (1985), Diagrammy Sostoyaniya Metallicheskikh System, ed. N. V. Ageeva, VINITI, Moskau, 30, 323). Chakrovorty et al. have indicated a miscibility gap between AlRu and AlNi up to high temperatures and have recorded the existence of a ternary equilibrium state β1–β2-γ′ (cf. in this respect Chartavorty S., West D. R. F., (1985), Scripta Metall., 19, 1355; Chartavorty S., West D. R. F., (1986), J. Mater. Sci., 21, 2721).
More recent works have described a continuous solid solution between AlRu and AlNi phases, although all specimens produced (using the casting process) had two phases, and annealing was in each case carried out for only a short time (less than 48 hours) (cf. in this respect Homer I. J., Hall N., Cornish L. A., Witcomb M. J., Cortie M. B., Boniface T. D., (1998), J. Alloys and Compds, 264, 173; Homer I. J., Cornish L. A., Witcomb M. J., (1997), J. Alloys and Compds, 256, 221; Hahls J., Cornish L. A., Ellis P., Witcomb M. J., (2000), J. Alloys and Compds, 308, 205). The authors have explained the two phases in the specimen on the basis of a so-called “coring effect”, which is supposed to occur on account of the different crystallization initiation for phases with very divergent solidus temperatures. This effect is supposed to be revealed by unclear boundaries between the grains and by the variation in the grain compositions.
Oxidation-resistant coatings based on nickel aluminides (nickel-aluminum alloys) which have been modified by precious metals, preferably by platinum, are already known (cf. in this respect Wolff I. M., (2002) in Intermetallic Compounds, ed. Westbrook J. H. and Fleischer R. L., 3, 53; Datta P. K., (2002) in Intermetallic Compounds, ed. Westbrook J. H. and Fleischer R. L., 3, 651; U.S. Pat. No. 4,447,538; U.S. Pat. No. 5,763,107; U.S. Pat. No. 5,645,893; U.S. Pat. No. 5,667,663; U.S. Pat. No. 5,981,091; U.S. Pat. No. 5,942,337; Lamesle P., Steinmetz P., Steinmetz J., Alperine S., (1995), J. Electrochem. Soc., 142(2), 497). The main advantage of coatings of this type compared to MCrAlY bond coatings (BC) is the improved bonding of the outer layer of a thermally insulating oxide (normally yttrium-stabilized zirconium, YSZ) to the metallic substrate. This bonding is effected by the formation of a pure, slow-growing α-Al2O3 layer at the interface between YSZ/BC during operation. Unfortunately, the platinum group metals which can be used as materials for thermal barrier coatings (TBC) have some drawbacks. These include, for example, the very high price of the metals, the depletion of aluminum in those interlayers which are enriched with precious metal, the high vapor pressure of the metals and volatility of the oxides which primarily occur (platinum dioxide, rhodium dioxide, etc.). This leads to a significant loss of the valuable metals during exposure to the aggressive, oxidizing environment at high temperatures (cf. in this respect Datta P. K., (2002) in Intermetallic Compounds, ed. Westbrook J. H. and Fleischer R. L., 3, 651; Kofstad P., (1988), High temperature corrosion, Elsevier applied science publishers LTD; Jehn H., (1984), J. Less-Common Met., 100,321).
Oxidation-resistant alloys which contain ruthenium have also already been investigated, and corresponding coating methods have already been developed (cf. in this respect U.S. Pat. No. 4,980,244; U.S. Pat. No. 4,759,380). The alloys studied probably consisted of a mixture of B2-CrRuAl and CrRuFeAl phases and an Ru- or Cr-based solid solution. A separate α-Cr phase was assumed in some compositions. The authors investigated CrRuAl alloys within the composition range from 0 to 60 atomic % in chromium, where the Ru:Al ratio was kept at approximately 50:50 in CrRuFeAl alloys. The best oxidation properties were discovered for alloys with a chromium content of between 30 and 60 atomic %. The CrRuFeAl and CrRuAl alloys have already been proposed as high-temperature protective coatings for different materials: superalloys, refractory metals or alloys with a refractory metal base, titanium or niobium aluminides, etc. Another possible application for Ru-containing phases is as a diffusion barrier for TBCs in order to prevent interdiffusion between the substrate and the bond coat (BC) (cf. in this respect U.S. Pat. No. 6,306,524). CrRuFeAlY alloys containing structural high-temperature materials have likewise already been investigated (cf. in this respect U.S. Pat. No. 6,306,524).